1. Field of the Invention
The present invention is directed to a method in the form of a pulse sequence for operating a nuclear magnetic resonance tomography apparatus, and in particular to a method suitable for use in the known gradient echo sequence.
1. Description of Prior Art
Nuclear magnetic resonance tomography devices (also known as magnetic resonance imaging devices) operate on the principle of exciting nuclear spins in a slice of an examination subject by means of an excitation RF pulse generated in the presence of a slice selection gradient in a first direction. The excited spins are phase-coded in a second direction, perpendicular to the first direction, by a phase-coding gradient. The resulting signal is read out under the influence of a read-out gradient in a third direction perpendicular to the first and second directions.
A problem in the operation of known magnetic resonance imaging devices shall be described with reference FIGS. 1 and 2. The slice of the examination subject is generally excited by generating a frequency-selective RF pulse in combination with a magnetic field gradient. The RF pulse generally corresponds to a "sync pulse" which deflects the nuclear spins by a flip angle .alpha. which is dependent on the position of the nuclear spins in the slice selection direction z. An exemplary curve for the flip angle .alpha. dependent on the z-coordinate in the slice selection direction is shown in FIG. 1.
Given fast pulse sequences, wherein the repetition time T.sub.R of the excitation RF pulses is shorter than the longitudinal relaxation time T.sub.1, the cross-magnetization which occurs in the steady state, and which contributes exclusively to the nuclear magnetic resonance signal, is dependent, among other things, on the flip angle .alpha.. There is thus a flip angle .alpha..sub.E, dependent on the ratio T.sub.R /.sub.1, at which the signal intensity becomes maximum. This flip angle is known as the Ernst angle and is dependent on the ratio T.sub.R /T.sub.1 in accord with the following relationship: EQU .alpha..sub.E =arc cos (exp T.sub.R /T.sub.1).
The obtainable signal intensity S at a flip angle .alpha., and at the repetition time T.sub.R and at the longitudinal relaxation time T.sub.1 is derived according to the following equation: EQU S.apprxeq.sin .alpha.(1-exp(-T.sub.R /T.sub.1)(1-exp(-T.sub.R /T.sub.1)cos .alpha.).sup.-1.
Because, as shown in FIG. 1, the flip angle .alpha. within the selected slice is dependent on the z-coordinate, the constant signal across the selected slice is not achieved in accordance with the equation immediately above, even given a uniform distribution of the nuclear spin density. According to equation immediately above, to the contrary, the shape of the slice profile is dependent on the selected, maximum flip angle .alpha. and on the ratio T.sub.R /T.sub.1. Slice profiles which, as desired, correspond to the Fourier transformation of the excitation RF pulses are excited for small flip angles .alpha., or for a high T.sub.R /T.sub.1 relationship. The signal contribution of the slice edges becomes greater than that of the slice center at a large flip angle .alpha.. FIGS. 1 and 2 show this case. The maximum flip angle .alpha. lies in the middle of the slice over a flip angle .alpha..sub.E (Ernst angle) which is assumed to be 20.degree., and at which the cross-magnetization, and thus the signal intensity, become maximum.
Related thereto, FIG. 2 shows the signal curve S over the z-coordinate in the slice selection direction for a uniform nuclear spin density. The aforementioned effect of the signal exaggeration at the slice edges thus becomes clearly visible. The maxima of the signal are reached at those z-coordinates at which the flip angle .alpha. is equal to the Ernst angle .alpha..sub.E, i.e. at 20.degree. in the illustrated case.
The flip angle .alpha. is greater than the Ernst angle in the middle the slice, so that a lower signal appears at that region.
As stated above, the signal is dependent not only on the flip angle .alpha. but also on the ratio T.sub.R /T.sub.1. If noticeable differences in the longitudinal relaxation time T.sub.1 occur within the tissue under examination, the shape and the half-width value of the slice profile within this tissue fluctuate with the longitudinal relaxation time T.sub.1. In materials having long T.sub.1, noticeably thicker slices, with distorted slice profiles, are excited than are excited in materials having a short T.sub.1. This proves a series problem in, for example, examinations of the head, wherein great differences in the longitudinal relaxation time T.sub.1 occur.